In high performance liquid chromatography, the commonly used packing materials are modified silica gels. Silica gel is generally modified by silylating it with silanes having various chemically modifying groups such that the chemically modifying groups are chemically attached to silanol groups on the surface of silica gel. Most widely used among others are octadecylated silica gel (ODS) packing materials which have octadecyl groups incorporated on the silica gel surface using octadecylchlorosilanes. Other known chemically modifying groups include octyl, butyl, methyl, cyanopropyl, and phenyl groups.
On actual use of the chemically modified silica gel packing materials in high speed liquid chromatography, problems arise with superfluous silanol groups remaining on the silica gel substrate. More particularly, polar solutes, especially basic solutes strongly interact with the residual silanol groups. This interaction prevents or retards elution of the basic solutes and causes peak tailing, so that reproducible chromatograms may not be obtained.
In order to reduce the influence of residual unreacted silanol groups, it was proposed to end cap the silica gel having chemically modifying groups attached thereto by subjecting the silica gel to silylation again with trimethylchlorosilane or the like for chemically attaching short chain alkyl groups to the residual silanol groups. The conventional end-capping technique is to subject the once modified silica gel to secondary silylation in a solvent such as toluene. The end-capped silica gel, however, retains more or less silanol groups unchanged, causing peak tailing.